Molecular analysis of the genetically programmed process of plant senescence

Abstract

Plant senescence is typically characterized by a sequence of biochemical and physiological degenerative events that result in the recycling of nutrients from the senescing tissues to developing organs of the plant. Molecular studies indicate that changes in gene expression are associated with the process of senescence. Genes specifically induced in expression during senescence include those encoding degradative enzymes, enzymes involved in pigment and phenolic biosynthesis, and enzymes involved in hormone biosynthesis, among others.^ Senescence of soybean cotyledons has been characterized and now serves as an excellent model system to identify genes involved in the initiation and subsequent progression of senescence. Reversal of senescence ("rejuvenation") in soybean cotyledons can be induced by removal of the epicotyl just below the primary leaves ("decapitation"). Using a subtractive hybridization protocol, cDNA clones from both senescent and rejuvenated cotyledons representing genes that exhibit up-regulation in expression during these developmental processes have been isolated and characterized.^ To investigate if the clones isolated from soybean are also involved in flower petal senescence, their pattern of expression has been analyzed in orchid flowers. This analysis has led to the identification of potential transgenes which could be introduced into orchid plants, using gene transfer methodologies, to extend the life of the flower for this important ornamental plant.^ A system for the genetic transformation of orchids has been developed. The protocol relies on the bar gene as a selectable marker since it confers resistance to the broad spectrum herbicide phosphinothricin (PPT) which allows for the easy selection of transformed plant tissues. Microprojectile bombardment of bar coupled with selection of bombarded tissues on bialaphos was found to be suitable for the transformation of three diverse orchid genera, Doritaenopsis, Cattleya, and Brassia, as illustrated by the recovery of bialaphos-resistant transgenic plants. With a system for selection and identification of transformed tissues in place, it is feasible to introduce economically important genes into orchids via co-transformation of bar with a second gene to modify commercially important traits. ^